Certain precast concrete products are produced using prestressing where high-strength steel tendons or strands are stretched between fixed points utilizing a calibrated tensioning mechanism. Concrete is then carefully placed and consolidated around the tendons. After the concrete hardens, forms are removed and the tendons are slowly detensioned thereby transferring the prestressing forces within the tendons to the concrete itself. The concrete is therefore compressed giving it much greater load-bearing capacity than nonprestressed units of the same configuration.
Types of structures which are currently built using prestressed products include bridges, stadiums, parking garages, warehouses and high rise buildings. Prestressed manufactured concrete is available in a wide variety of cross-sectional shapes. Types of precast concrete products which are produced using this technique are beams, bulb-tees, piling, double tees, inverted tee beams and columns. The applications where prestressed precast concrete is a viable option will undoubtedly increase, due to price increases of competing products like structural steel.
Prestressed concrete products are increasingly being designed with higher concrete strengths and greater quantities of tendons. This development has allowed greater product lengths, which both reduces overall construction costs and shrinks the time required to complete a project. Since both of these end results are very important to owners, concrete strengths and tendon densities continue to increase. Recent technological developments, like self-consolidating concrete, have given prestressed concrete producers the confidence to aggressively pursue these more demanding projects, taking away market share from competing products.
Prestressed concrete products are typically cast end to end in beds ranging from 200′ to 600′ in length. A full-length soffit forms the underside of the product and removable side forms give the proper shape to the product's sides. Endplates or headers form the ends. Side forms are pulled tightly against the soffit to eliminate grout leaks and are kept the proper distance apart at the top using crossties.
Endplates form the end of a single prestressed member. Constructed out of steel or plywood, solid endplates function similarly. Steel endplates are more expensive than plywood, but can last for years, whereas inexpensive plywood endplates may only last two to five pours. In order to shorten the cycle time and reduce labor costs, some producers have chosen to use three-piece slotted endplates. Slotted endplates allow a producer to tension the bottom flange strands before locating endplates on the bed, shortening the production cycle. The end of the resulting beam is ragged, however, and may require labor intensive remedial work before it can be shipped.